Lift / Drag curves - NACA 8-H-12 airfoil [Archive]

gyrogreg

06-10-2005, 09:26 AM

Can anyone tell us where on the internet we could find the L/D curves for the NACA 8-H-12 airfoil?

An Engineer in France, and a new gyroplane pilot, is asking for more legible data than he can read in the material available to him. As ussual with people recently introduced to gyros, he is fascinated with principles and as an intellectual excercise, is writing a program to study autogyro blade motion. I figure any of his work would be extremely interesting to the people on this forum.

thanks, Greg Gremminger

Victor Duarte

06-10-2005, 10:18 AM

Hello Greg,
i am interested also to know who is this person. I wrote a gadget that performs lift drag calculations on a blade and i would love to know what this person is working on.
An interesting and easy to handle software that gives those curves is NVFoil.
http://www.geocities.com/CapeCanaveral/Hangar/2524/nvfoil/nvfoil.html

cheers

twistair

06-10-2005, 11:15 AM

http://www.mh-aerotools.de/airfoils/javafoil.htm

If your guy is tough serious it also may worth to surf NACA digital library at http://naca.larc.nasa.gov/ using their searh tool for reports containing words "gyroplane" etc (I recall there is detailed study of subject airfoil). If he is not - then attached pic may be enough. It has C.Beaty comment and was posted somewhere at the old forum.

Cheers,

C. Beaty

06-10-2005, 07:26 PM

gyrogreg

06-11-2005, 02:53 AM

Here is the email request I received from Mike Goodrich (mike.goodrich@total.com)
in France------------------

Greg

I came across your site via the Magni website. I'm in France and have recently been checked out in a Magni M16 by Eric Changeur (the French agent for Magni).

As an engineer I'm fascinated by these machines and am trying to develop a more powerful and better engineered pre rotating system with the help of Eric.
For this I need the Lift /drag curves for the NACA 8-H-12 aerofoil. I'm also writing a program for myself to study the blade motion of an autogyro, I'm sure it's been done before but it's a challenge I've set myself out of pure intellectual curiosity and I need thee curves for thatas well.

I've lacated the official curves on the NACA website but they arevery poor scanned curves and unusable.

Do you have or know anybody who has a clean set of these curves that I could get a copy of?

I have a software that allows me to load a scanned curve and it then plots the x and y values pixel by pixel giving an Excell type table for each curve with about 1000 points per curve. If you can find me a clean copy I'd be happy to supply whoever has it with the tabulated values once I've finished.

I hope you can help and congratulate you on the articles you've written I've found them very instructive.

Mike Goodrich
Senior Rotating Machinery Engineer
Total France

Mike G

06-23-2005, 07:31 AM

Thank you all for the replys so far.
The first challenge I've set myself is to try to develop a better pre rotator than the slipping fan belt clutch and flexible drive curently on the Magni.
For this I need to calculate the torque required to accelerate the inertia of the rotor plus the torque required to overcome the ever increasing drag of the rotor as it accelerates.
This would be quite simple if I had a decent Cd curve of the 8 H 12 blade that Magni use. I need Cd against angle of attack for different Reynolds numbers and I found exactly what I needed on the NACA website. They are scanned copies of the original wind tunnel tests back in 1949 but they are unreadable. Does anybody have a decent copy of these curves?

I looked at the curves posted as replies to my initial question and wonder how to use them because only the pitching moment seems to be plotted against angle of attack. Could someone please explain how this format of lift/drag curves works?
I was interested to see that the two examples given were quite different with the Cl rising to 1.3 in one curve and only 0.65 in the other. Is this due to assumed roughness of the aerofoils?

Mike G

C. Beaty

06-23-2005, 08:48 AM

I did both of those plots, Mike.

The first with Cl max of 0.65 was using the original NACA published ordinates which are quite lumpy.

The second was with the ordinates smoothed and evenly spaced.

The original NACA data was taken at low speed in a wind tunnel with essentially zero turbulence at a Rn of 2.6 * 10^6.

I do have the drag polars as reproduced in a Federal publication but they’re not very legible either. I can attempt to scan them in high resolution and E-mail them to you if you like.

A word of caution: You will delude yourself if you take the NACA drag curves as gospel. NACA’s low turbulence wind tunnel and the real world are 2 different places.

If you assume the mean profile drag coefficient to be 0.01 and the lift coefficient to be 10% of the angle of attack in degrees, you’ll be nearer the truth.

Sorry I neglected including lift Vs. angle of attack in the original post.

Mike G

07-01-2005, 05:07 AM

Mr. Beaty
You'll have to tell me what the "C" stands for, calling you Mr.Beaty is like talking to one of my father's friends.
Thanks for the feedback and the advise re inaccuracies . As an old time engineer I only use precise calculations as the best centre point I can find for my very large tolerance band.
I'd be greatful for any data you can e:amil to me at mike.goodrich@total.com
. I'm out of the office a lot so replies from me will be pretty sporadic.

Does anybody out there have any coast down data (rpm v time) for a Magni M16 rotor? Knowing the inertia I could back out a Cd value and that would give me a bit more confidence in my calculations for the actual rotor I'm considering.

Does anybody have any data or calculations regarding the power needed to accelerate a rotor during pre rotation? I'd like to see if one of you has a better method than I do.

Mike G

gyrogreg

07-01-2005, 05:14 AM

Does anybody out there have any coast down data (rpm v time) for a Magni M16 rotor? Knowing the inertia I could back out a Cd value and that would give me a bit more confidence in my calculations for the actual rotor I'm considering.

Mike,

I'll try to get that number for you - maybe today. - Greg

David

07-09-2005, 07:42 PM

Can anyone tell me what is a good place to start in respect to the angle of attack for the 8 H 12 blade? Dave N

gyrogreg

07-10-2005, 04:01 AM

Mike,

I'll try to get that number for you - maybe today. - Greg

Mike,

Sorry I did not post these spin-down times earlier. I had tested it that day, but forgot to post it 'till now.

I did this twice. Immediately after landing, I started timing the spind-down at 300 RPM: Density Altitude was about 3000 ft MSL. Was don with low winds and with rotor disk held flat - no wind through the blades to affect their RPM.

Time from 300 RPM to 200 RPM: 10 seconds
Time from 300 RPM to 100 RPM: 32 seconds

My thoughts:
Above 200 RPM, aerodynamic drag is most prevelant for spin-up. Below about 150 RPM, aerodynamic drag seems to be minimal to power required for spin-up. I'm not sure what this can tell you about inertia of blades - depends on aerodynamic and other drags, which are not really measured. Can the curve of the spin-down tell you something about the two drags and allow you to compute inertia?

Thanks, Greg

Mike G

07-11-2005, 01:01 AM

Greg
I'm back in the office for a few days. Thanks for the data, I'll look at it when If read all my mail.

Mike G

Ted Eggleston

07-11-2005, 01:57 AM

Greg,

With the increase in rpm's comes an increase in the coning angle and I suspect a slight increase in the angle of attack. Will this make it difficult to plot the drag co-ordinates?

Ted

Mike G

07-12-2005, 07:42 AM

Greg
I plotted the coast down of your Magni rotor assuming 2 x 18.4 kg (40.5lb) blades with a 2 meter (78.75 ins) CG from the centre of the rotor. This gives me an inertia of 147.2 kgm² (3485 lb.ft²) (although I haven't included the mass of the blade carrier and bolts, if you happen to have it it would tidy up the calculation). I then ask Excel to tell me what Cd I need to get 10 secs coast down time from 300 to 200 rpm and I get 0.0162 and this then gives me 42 secs to go from 300 to 100 rpm.

The 0.0162 seems a bit high compared to the 0.01 value Chuck gave in an earlier message but it's probably because as the rotor slows down the Reynolds number drops and the Cd increases so I would expect the 42 secs to be nearer your 32.

This shows I'm in the ball park but I'd like to refine the calculation a bit more so I'm still in the market for a set of Cx/Cd curves for different Reynolds numbers if anyone out there can help I'd be greatfull.

Ted
I'm not sure I see how the coning angle would change the angle of attack can you educate me on that one, I'm obviously missing something in my understanding of how a rotor works.

Mike G

PS How do I paste a copy of my excel graph into a message?

gyrogreg

07-12-2005, 08:42 AM

Greg
--- I haven't included the mass of the blade carrier and bolts, if you happen to have it it would tidy up the calculation ---

Mike. I don't know the mass of the hubbar exactly, but it is steel and I would guess that it weighs about 8 Lbs. It is about 18 inches in diameter. these are not exact measurements and I will not be able to get better measurements for several weeks - getting ready for Mentone and Oshkosh!

Ted
I'm not sure I see how the coning angle would change the angle of attack can you educate me on that one, I'm obviously missing something in my understanding of how a rotor works.

Mike, I don't see how the coning angle can change the AOA of the blades. Besides, the Magni blades are very stiff and do not cone or droop much.

PS How do I paste a copy of my excel graph into a message?

Mike, I've never used the attachments option, but there is an attachment option under the "Additional Options" when you use "QUOTE" to reply to a post. I'll try a test attachment of the Magni USA logo.

wisemo

10-27-2008, 08:39 AM

The 8H12 isn’t the greatest rotorcraft airfoil but it’s not as bad as those early plots of mine look.

The NACA ordinates aren’t smooth or evenly spaced which causes some strange results with a computer airfoil program.

Attached are the results when the ordinates are smoothed and evenly spaced. Not great but better.

Hi Beaty,

Do you have a new set of smoothed ordinates for the airfoil? If you do could you please send them over? Would be greatly appreciated :)

Mo

C. Beaty

10-27-2008, 09:00 AM

8H12 smoothed

1.00000000 0.00000000
0.99726094 0.00006382
0.98907372 0.00025692
0.97552803 0.00058338
0.95677225 0.00103403
0.93301189 0.00162158
0.90450728 0.00313926
0.87157076 0.00656664
0.83456323 0.01245016
0.79389022 0.02004169
0.74999742 0.02852761
0.70336580 0.03771430
0.65450633 0.04754265
0.60395441 0.05758942
0.55226392 0.06730854
0.50000123 0.07621519
0.44773896 0.08378481
0.39604966 0.08970061
0.34549959 0.09312369
0.29664252 0.09379994
0.25001361 0.09121309
0.20612360 0.08615830
0.16545318 0.07941077
0.12844777 0.07131371
0.09551263 0.06207947
0.06700841 0.05226471
0.04324722 0.04200000
0.02448924 0.03180000
0.01093985 0.02180000
0.00274741 0.01237393
0.00000000 0.00000000
0.00273314 -0.00350656
0.01091483 -0.00904840
0.02445659 -0.01289156
0.04320988 -0.01594237
0.06696905 -0.01822117
0.09547356 -0.02001752
0.12841089 -0.02153174
0.16541994 -0.02268064
0.20609503 -0.02356811
0.24999031 -0.02419474
0.29662469 -0.02453091
0.34548709 -0.02478912
0.39604207 -0.02491327
0.44773565 -0.02476465
0.50000145 -0.02432315
0.55226681 -0.02360209
0.60395914 -0.02272256
0.65451212 -0.02158700
0.70337195 -0.02000412
0.75000338 -0.01823750
0.79389558 -0.01643242
0.83456773 -0.01437451
0.87157428 -0.01217171
0.90450983 -0.01006756
0.93301357 -0.00780289
0.95677323 -0.00541821
0.97552849 -0.00313998
0.98907388 -0.00141271
0.99726096 -0.00035484
1.00000000 0.00000000

wisemo

10-27-2008, 09:14 AM

Thank you so much! (trying to find a kiss emo-icon LOL)
Mo

Gyro_Kai

10-27-2008, 10:13 AM

The 8H12 isn’t the greatest rotorcraft airfoil but it’s not as bad as those early plots of mine look.

Hello Chuck (which I believe is Mr. Beaty's first name :D)

Your statement suprises me. I thought more or less all modern blades uses this profile. Are there better ones around?

Kai.

C. Beaty

10-27-2008, 11:49 AM

The NACA 8H12, Kai, was designed in the late 1940s in an effort to improve the performance of early helicopters. It did not.

It was conceived at the very beginning of laminar airfoil development before the practical limits were fully understood, using a wind tunnel of very low turbulence. A rotor is anything but a low turbulence environment.

When laminar flow breaks down, there is an abrupt increase of drag and decrease of maximum lift.

The only advantage of an 8H12 is ease of hand starting. The stall is gradual rather than abrupt like in had been the standard rotorcraft airfoil, the symmetrical NACA 0012.

Modern helicopter airfoils, such as the Boeing VR-7 are much better than the 8H12 and are equally easy to hand start.

As in most things connected to gyros, Bensen set the pattern and the derivatives have followed.

magilla

10-28-2008, 03:33 PM

Chuck,

Please forgive my ignorance, but aren't helicopter airfoils completely different because of the fact they are driven by a mast?

Aren't the driven, driving, and stall regions a little different between the two types of airfoils?

I guess it boils down to this: are you saying a VR-7 blade structure of appropriate size for the gross weight of a gyro would perform better than a NACA 8H12 of same size?

C. Beaty

10-28-2008, 05:14 PM

Take a look at the images, Spencer. There is a difference between an 8H12 and a VR-7 but it has nothing to do with helicopter vs. gyro. The 8H12 was originally conceived as a helicopter rotor airfoil; that’s the reason its middle name is “H.”

The A&S 18A uses an NACA 0012 rotor derived from the Hiller helicopter. The J-2 uses an NACA 0015 airfoil that came from the Hughes 269.

There’s no difference between helicopter airfoils, gyroplane airfoils and wind turbine airfoils except direction of twist. That’s because stall begins at retreating blade tips on helicopters and at root ends on windmills and gyroplane rotors.

magilla

10-29-2008, 02:16 AM

Aaaaaaah, I see: So it's not the airfoil, but the TWIST you have to get right for the gyro blades.

Has anyone determined the optimum twist angle for a gyro blade of x length?

So that's why you can't just bolt on a set of helo blades....

Gyro_Kai

10-29-2008, 03:01 AM

C. Beaty

10-29-2008, 03:34 AM

Symmetrical airfoil helicopter blades can be inverted and spun backwards; that puts the twist in the right direction. And even without being inverted, helicopter blades still autorotate as you well know.

I once bought a truckload of Hughes 269 (TH-55) and OH-6 blades from Ft. Rucker, mostly runouts but a few that had been pulled for one reason or another, for scrap prices and nearly everyone in Florida flew on these blades for years.

Hughes blades have a built in twist of 8º. Raoul Hafner, A British rotorcraft pioneer, calculated optimum Autogiro twist to be 3º between 30% and 100% radius in a paper published by the Royal Aeronautical Society.

C. Beaty

10-29-2008, 12:26 PM

Chuck,

thanks a lot. To the naked and untrained eye the two profiles look almost the same, could you explain a bit more to the laymen what the significant differences are?
:hail:

Is any blade on the market using this advanced profile?

Magilla,

please note, also helicopter blades are sometimes twisted, but the other way around:

For Gyros it is good to have a higher pitch on the outside than inside. The inside portion of the blade is to provide drive so it could even be negative, to reduce the stall area and have more drive area. Outside the twist could be more upward to provide more lift.

In helicopters with straight blades, you have an increasing lift towards the outside as it has a higher airspeed. So it makes sense to give the inner part of the blade an upwards pitch towards the outside, so that the inside of the blade produces a lot of lift with a little airspace, spreading the load more over the length of the blade.

I believe there are gyros around with heli-blades mounted inside-out and turning the other way.

Kai.The 8H12, Kai, was designed for a greater extent of laminar flow than can be achieved in practice. This leads to early stall and limited maximum lift coefficient.

The 8H12 has a published maximum lift coefficient of 1.2 while a Boeing VR-7 has published max. Cl of 1.6.

The lift/drag ratio of a gyroplane rotor can be determined by measuring the rotor disc angle of attack. It is approximately equal to 1/tanθ, theta being rotor disc angle of attack. If, for example, the disc flies at an angle of 10º, the L/D ratio is 5.67:1.

I made some measurements on a few rotors a number of years ago. A straightedge was pivoted to a post and while a gyro flew past at a constant 50 mph, the straightedge (a board) was aligned with the rotor disc via a pair of draw strings and the angle measured with an electronic level.

DWs were the lowest drag at ~8:1, Skywheels (8H12) were next at ~7:1 and the rest of the pack, Bensens, Rotordynes, etc ranged around 5:1.

The attached photo is a VR-7 rotorblade I built quite a few years ago for use on a 3-blade gyro with a 6” chord. It’s beginning to look a bit scruffy after 10-15 years in my barn.

Gyro_Kai

10-29-2008, 01:32 PM

Thanks a lot Chuck.
DW are by far the best renowned blades, if I do a survey across this forum, what profile do they use?

Kai.

C. Beaty

10-29-2008, 02:24 PM

This is about as near as I can come using a standard airfoil program.

DWs use the thickness distribution of standard NACA 4 digit airfoils and approximate a NACA 25012 meanline with 5% of the trailing edge reflexed for zero pitching moment coefficient.

Gyro_Kai

10-29-2008, 10:42 PM

Entire thread copied and archived. Not yet fully understood, though.:der:

After reading through Charnov and a couple of Heli-Theorie books, the next on my list will be Aerodynamics and airfoils.

Can anyone recommend good books for someone with non aviation engineering background?

Kai.

C. Beaty

10-30-2008, 06:13 AM

Theory of Wing Sections by Ira H. Abbott and Albert E. von Doenhoff is a good starting point.

It’s available in paperback from Dover Publications.

Gyro_Kai

10-30-2008, 06:30 AM

Thanks a lot!

wisemo

10-30-2008, 07:28 AM

Chuck, this information you're giving us is gold. Thanks... every little helps.

Xavier AVERSO

10-30-2008, 08:04 AM

quadrirotor

10-30-2008, 09:39 AM

J'ai volé avec deux rotors VR7 c'est vrai que ce profil est plus performant que le 8H12 en portance et en finesse, mais il est plus instable et l'autogire devient difficile a piloter.

J' flew with two rotors VR7 c' is true that this profile is more powerful than the 8:12 in bearing pressure and smoothness
but it is more unstable and l' autogiro becomes difficult has to control.

Valuable experience! Expérience inestimable!

Ideal gyro blade:
http://www.rotaryforum.com/forum/showthread.php?t=3474

VR7 equiped: Destroyed
http://www.rotaryforum.com/forum/attachment.php?attachmentid=7988&d=1107876671

C. Beaty

10-30-2008, 09:45 AM

There are two important criteria for rotor stability and control, Xavier.

1) Zero pitching moment coefficient. An airfoil, if cambered (curved), tends to pitch nose down unless compensated by a reflexed trailing edge. The trailing edge tab of the VR-7 is an integral part and must be set at an angle of –5.9º in order to obtain a zero pitching moment over the mach number range of interest. The attached table of ordinates gives the correct dimensions.

2) Chordwise balance. The CG of rotor airfoils should be at 25% of chord from the leading edge.

If these criteria are met, all rotorblades having the same moment of inertia and chord will have about the same handling and control qualities.

Data reproduced here is from a Boeing document.

Xavier AVERSO

10-30-2008, 02:01 PM

Merci André pour la photo de l'autogire d'yves Megret prise au Bois de la Pierre, c'est avec des rotors VR7 fabriqués par Megret que j'ai volé.
rotors très léger en composite de 18 cm de corde et bien équilibré a 25% de la corde,yves étant un scientifique je suppose qu'il a bien réalisé le profil.
Il serait interessant d'avoir l'avis d'autres pilotes ayant volé avec un rotor bipale VR7 pour comparer 8H12 ?

Thank you Andre for the photograph for l' autogiro d' Yves Megret taken with the Bois de la Pierre, c' is with rotors VR7 manufactured by Megret that j' flew. rotors very light in composite of 18 cm chord and balanced well has 25% of the chord, Yves being a scientist I suppose qu' he carried out the profile well. He would be interesting d' to have l' opinion d' others pilot having flown with a two-bladed rotor VR7 to compare 8H12 ?

wisemo

11-01-2008, 07:29 AM

C. Beaty

11-01-2008, 12:03 PM

chuck - does airfoil affect the angles of articulation required? meaning while manoeuvring the hub needs to be tilted forward,backward, left and right. these tilt angles- do they change much depending on airfoil? what are typical angles?No. The magnitude of a rotor’s thrust varies only slightly with lift/drag ratio; the magnitude of rotor thrust being the square root of the sum of squares of lift and drag. There’s ~1.5% difference in thrust magnitude between a rotor with 10:1 L/D and one with 10:2 L/D.

Control by a rotor with central (teetering) flap hinges requires the rotor thrust to be vectored about the CG to produce a control moment. This begins an angular acceleration in pitch or roll proportional to airframe moment of inertia/torque; torque being the product of rotor thrust and offset relative to CG.

The low drag rotor may produce the feel of greater control sensitivity in forward flight. The low drag rotor (10:1) flies at a disc angle of attack of 5.7º while the high drag rotor (10:2) flies at a disc angle of attack of 11.3º. One might say, as an approximation, the weight of the machine is contained within a 5.7º envelope while the other is contained within a 11.3º envelope.

I’ve heard individuals complain that low drag rotors produced the impression of greater control sensitivity but the bottom line is that a given control deflection produces nearly the same control moment.